Add stub OSXIsGUIApplication for non-mac platforms

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxPython/Phoenix/trunk@75485 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Only build the webkit module on Darwin

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxPython/Phoenix/trunk@75486 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775